A vehicle body fully made of aluminum is known to achieve weight reduction to improve fuel efficiency and motion performance of a vehicle. However, the weight reduction achieved by such an all-aluminum vehicle body accompanies a considerable rise in production costs. In recent years, a method of designing and manufacturing so-called multi-material vehicles has been attracting attention as a way of reducing weight of vehicles without a large increase in cost, and production of such vehicles has already been performed. In the designing and manufacturing method, steel and other lightweight materials, such as aluminum, magnesium, or resin products including fiber-reinforced plastic (FRP) and carbon fiber reinforced plastic (CFRP), are combined in a single vehicle.
In the case where the same steel materials as in the conventional vehicle bodies are to be used for the most part of the framework of a vehicle body and an aluminum panel is to be partly used, it is necessary to join dissimilar metals together. For example, when a roof, an upper back, and the like made of aluminum alloy are to be joined to a steel framework, joining dissimilar metals together, i.e. joining an aluminum panel member to the steel framework is necessary.
Rivets such as self-piercing rivets, blind rivets, bolts, punching screws, caulking joints, or the like are used to form such a joining portion of dissimilar materials. However, such joining techniques all belong to spot joining and require an additional waterproofing process using a suitable sealing resin agent or the like when applied for an outer plate part, a floor part, a chassis part, or the like which is exposed to rainwater. In addition, applying an adhesive is applied between the members can be required to prevent corrosion.
JP 2014-73798 A relates to dissimilar metal joining for joining an aluminum panel member to a steel framework, and discloses joint structure for an automobile in which a roof panel made of aluminum alloy is joined to a steel framework member by melt-bonding. The dissimilar metal joining of JP 2014-73798 A employs linear joining to prevent infiltration of rainwater and to ensure strength reliability.